Dedifferentiation and enhanced proliferation and motility of vascular smooth muscle cells (VSMCs) are key elements in the pathogenesis of many vascular diseases. Thus, a detailed understanding of the development of the VSMC layer of normal blood vessels promises to provide critical insights into vascular disease. However, the processes underlying the morphogenesis of any specific vessel or vascular bed are not well understood. The central goal of this proposal is to elucidate the key molecular and cellular events encompassing the morphogenesis of the smooth muscle cell (SMC) layer of the left pulmonary artery during the initial four days of murine lung development. This goal is the first step towards the long term objective of delineating the critical events in the building of all layers of the pulmonary artery throughout development. An essential element of achieving these goals is to characterize the roles specific signaling pathways play in pulmonary artery development. Platelet derived growth factor (PDGF)-induced signaling has been implicated in the pathogenesis of pulmonary artery hypertension and restenosis following coronary artery angioplasty; yet, the specific effects of PDGF signaling in these diseases or in the morphogenesis of large vessels remain elusive. I hypothesize that signaling through the PDGF pathway is critical for the maintenance, proliferation and/or recruitment of a multipoint early lung mesenchymal cell population that has the capacity to differentiate into the SMC layer of the left pulmonary artery. The proposal has three specific aims: 1) establish a timeline of molecular markers and cellular events that identify specific stages during the transition of a murine lung mesenchymal cell into a differentiated left pulmonary artery SMC and relate this timeline to'endothelial cell and non-vascular tissue development; 2) identify the pattern and effects of activation of PDGF receptor-mediated signaling pathways in left pulmonary artery SMC development; 3) determine the fate in the lung of early PDGF receptor-beta-positive mesenchymal cells through lineage analysis. To achieve these aims, lungs will be dissected from wildtype and transgenic mouse embryos and subjected to whole mount immunostaining, in situ hybridization or X-gal staining. In selected experiments, embryonic lungs will be cultured prior to analysis. Confocal microscopy will be used to analyze fluorescent stains on a cellular level. In sum, the proposed work will make the left pulmonary artery the best understood example of morphogenesis of a specific blood vessel. These studies will yield critical insights into the mechanisms underlying prevalent vascular diseases such as coronary artery atherosclerosis. In addition, this work promises to advance the investigation of critically understudied disorders of smooth muscle cell physiology in the lung, such as pulmonary artery hypertension and lymphangioleiomyomatosis.